GeolCarp_Vol50_No1_49

GEOLOGICA CARPATHICA, 50, 1, BRATISLAVA, FEBRUARY 1999

49–61

ORIGIN OF RHYTHMICALLY BEDDED CENOMANIAN

CARBONATE ROCKS OF THE BAKHCHISARAI REGION

(SW CRIMEA)

RUSLAN GABDULLIN

, ANDREY GUZHIKOV

and IVAN DUNDIN

Moscow State University, Geological Faculty, Department of Historical and Regional Geology,

Vorobiovy Gory, 119899 Moscow, Russia; ruslan@geol.msu.su

Geological Institute of Saratov University, Moskovskaya 161, 410750 Saratov, Russia

Saratov State University, Geological Faculty, Department of Geophysics, Astrahanskaya 83, 410071 Saratov, Russia

(Manuscript received January 14, 1998; accepted in revised form December 11, 1998)

Abstract:

Rhythmically bedded marls and limestones were studied in the Cenomanian deposits of SW part of Mountain

Crimea in Ukraine. Carbonate content, TOC, XRD, foraminiferal, petromagnetic analyses and ichnofossil distribution
analysis were carried out on 115 samples from 6 sections to define the nature of 5 types of rhythmicity. Marl-sandy
marl rhythms of the Lower Cenomanian transform into limestone-marl rhythms of the Middle Cenomanian. Upper
Cenomanian rocks contain limestone-limestone, limestone-marl rhythms and rhythmically bedded black shales (analog
of “Bonarelli level”). The types of the rhythms can be classified due to lithology and their paleogeographic position.
Cycles of dilution, solution and bioproduction are involved, 11 paleogeographic models are discussed, 6 are proposed.
The nature of the rhythms can be connected with Milankovich cycles.

Key words:

Crimea, Cenomanian, rhythmicity, carbonate rocks, paleogeographic models.

rhythmically bedded rocks and paleogeographical models
of their origin were investigated by R.R. Gabdullin (Gab-
dullin 1997; Gabdullin & Baraboshkin 1997).

The following rhythms were observed: (1) Lower Cenoma-

nian: marl-sandy marl (Mantelliceras mantelli Zone, Inocera-
mus crippsi Subzone), (2) Middle Cenomanian: marl, marly
limestone-limestone (Rotalipora cushmani Zone) and (3) Up-
per Cenomanian: limestone-limestone and limestone-marl, in-
cluding rhythms in “black shale” (Rotalipora cushmani Zone,
Whiteinella archeocretacea Zone).

Cenomanian deposits on the Russian craton are usually

presented by mostly terrestrial (sands, sandstones) and rare
carbonate rocks. These deposits of variable thickness (0–
70 m) with extremely rare rhythmical bedding contain many
erosive surfaces and phosphate concretions. That is why we
decided to investigate the comparatively «complete»
Cenomanian sections of Crimea.

Methods of study

In the field the succession was divided into rhythms based

on the weathering profile (mostly Middle Cenomanian),
colour difference and ichnofossil distribution (mostly Middle
Cenomanian), thickness variation (exept Late Cenomanian)
and distribution of pyrite concretions.

Thin sections of 50 samples were studied. X-ray diffrac-

tion analysis (2 samples), foraminiferal analysis (4 samples),
total organic carbon content and calcium carbonate content
analysis (50 samples) were used. Petromagnetic investiga-
tions (115 samples) included measurements of magnetic sus-
ceptibility (k), natural remanent magnetization (Jr), remanent

Introduction

This study focusses on the depositional history of Cenoma-
nian rhythmically bedded carbonate rocks outcropping in
Crimea.

The most important biostratigraphical investigations in the

studied region were made by D.P. Naidin and A.S. Alekseev
(Naidin & Maslakova 1958; Naidin et al. 1975, 1980; Naidin
& Kyashko 1994). Now it is known that Cenomanian deposits
have erosional contacts both with the Upper Albian (Mor-
toniceras inflatum, Stoliczkaia dispar zones) and the Lower
Turonian (Whiteinella archeocretacea, Helvetoglobotruncana
helvetica zones). The Lower-Middle Cenomanian boundary is
erosional too. The thickness of the whole Cenomanian varies
from 20 to 70 meters.

Anoxic events are commonly observed near the Cenoma-

nian-Turonian boundary of this region. An example is a
“black shale” in the Aksu-Dere section, located in the stud-
ied area. This anoxic event was studied in detail by many sci-
entists (Naidin & Kyashko 1994; Gavrilov & Kopaevich
1996; Alekseev et al. 1997).

The Early Cenomanian was a time when a rapid trans-

gression took place. The depth of the basin constantly in-
creased during the Cenomanian up to 500 meters (accord-
ing to foraminiferal data, Dolitskaya 1972). During the
Cenomanian the development of the basin took place un-
der stable tectonic conditions. It must be noted that the
paleogeography of this region is poorly investigated. Au-
thors propose paleogeographic sketches for the Lower,
Middle and Upper Cenomanian.

The rhythmicity in the part of the Middle Cenomanian

was studied by V.T. Frolov (Frolov 1996). Cretaceous

50 GABDULLIN, GUZHIKOV and DUNDIN

saturation magnetization (Jrs), destructive field of rema-
nent saturation magnetization (H’cs) and magnetic sus-
ceptibility increase (dk). To determine mineral species
with magnetic properties, differential thermal magnetic
analysis (DTMA) was used, and JR-4 and IMB-2 ma-
chines were used for remanent magnetization and mag-
netic susceptibility analyses. The studied rocks are char-
acterized by extremely low size of natural magnetization,
which is close to the limits of error of the method: mag-
netic susceptibility varies from 1

–5

to 2

–5

SI (stan-

dard units) and natural remanent magnetization changes
from 0.005 to 0.05 nT (T=N/Am). The strenght of mag-
netic susceptibility and natural remanent magnetization
increase under the influence of laboratory magnetic field
and temperature (k from 5.9 up to 29.3 nT and Jr from 0

to 55

–5

). Increasing magnetic susceptibility is connected

with the thermal transformation of iron sulphides into magne-
tite. So the presence of pyrite and pyrrothine in rocks is proved
by magnetic susceptibility increase. Vertical graphs of mea-
sured parameters do not show cyclic changes but the calculat-
ed sizes of Jrs-Hcs correlation, variances of Jrs and H’cs have
rhythmic fluctuations (window size — 5 samples, step size —
1 sample). The use of petromagnetic methods can help scien-
tists to: (1) determine low concentrations of sulphide and non-
sulphide Fe-magnetics of dust size invisible even in thin sec-
tions; (2) to distinguish the composition and volume of the
terrestrial input; to understand the nature of magnetic miner-
als. Petromagnetic methods are good for subdivision and cor-
relation of studied sections. Cyclic distribution of magnetic
minerals detected by these methods can be interpreted as cy-

Fig. 1.

Geological sketch (a) and paleogeographical sketches (b–d) of Bakhchisarai region (SW Crimea) with studied outcrops. Legend: 1

— underlaying rocks; 2 — Cenomanian rocks; 3 — overlaying rocks; 4 — sections (location and number); 5 — direction of the
terrestrial input; 6 — depth of the basin (m); 7 — regime, conditions; 8 — proposed boundaries between paleogeographic areas.

ORIGIN OF RHYTHMICALLY BEDDED CENOMANIAN CARBONATE ROCKS 53

Stratigraphy

Lower Cenomanian rhythmically bedded marls contain

Puzosia planulata

(Sow.) and Inoceramus crippsi (Mantell).

This stratigraphic interval is characterized by the Mantel-
liceras mantelli Zone; Inoceramus crippsi Subzone or Thal-
maniella deckei Zone (section 1). Rare remnants of plants
and insects are known from these deposits.

Most parts of the Middle Cenomanian rhythmically bedded

marls and limestones (sections 1, 2) include the ammonites
Mesogaudryceras leptonema

(Sharpe), M. rarecostatum Balan,

Galycoceras(?)

sp., the bivalves Inoceramus virgatus Schlut. and

belong to the Rotalipora cushmani Zone (sections 1–3, 5).

Four petromagnetic complexes (PC) were determined by

complex analysis of petromagnetic parameters in the Middle
Cenomanian of Selbuhra Mountain (Cushmani Zone, Fig. 4).
A brief schematic description of them is given here (Table 1).
It should be noted, that the small quantity of samples in the
uppermost part and in the bottom part of the investigated sec-
tion results in an absence of calculated parameters. PC 1.
Marls usually contain pyrite concretions and ichnofossils Phy-
cosiphon

, Chondrites, indicating low content of oxygen in wa-

ter and rare Planolites. Limestones are characterized by ichno-
coenoses: Planolites, Zoophycos, Thallassinoides and
Teichichnus

PC 2.

Pyrite concretions and the ichnogenus

Chondrites

are absent. PC 3. Ichnocoenoses: Zoophycos, Te-

ichichnus

and rare Planolites are present. Deposits of PC 4

contain rare Teichichnus and Zoophycos. At the top of the
Middle Cenomanian a bentonite layer was observed. Rocks of
third and fourth petromagnetic complexes include pyrite con-
cretions.

The Upper Cenomanian in the studied sections is poor in

macrofossils: rare parts of the brachiopod Lingula belbeken-

sis

Klik. in section 3 (limestone couplets) and bivalves (inoc-

eramids) in section 4 (limestone-marl rhythms). Remnants of
deep marine fish were found in the black shales (Mazarovich
& Mileev 1989). Calycoceras naviculare Zone of the Upper
Cenomanian in the studied area was proved by D.P. Naidin
(Naidin et al. 1975).

Upper Cenomanian rocks of Selbuhra Mountain consist of

2 petromagnetic complexes. Graph of Jrs-H’cs variance can
be divided into 2 parts: lower (oscillation) and upper (no os-
cillation). Clear rhythmicity and the presence of the bento-
nite couplet is typical for the lower complex (PC 1). Depos-
its of the upper complex (PC 2) lose rhythmicity near the
Cenomanian-Turonian boundary (chaotic distribution of lay-
ers). The Upper Cenomanian rocks (Selbuhra section) are
characterized by medium Jrs-H’cs variance and low correla-
tion. H’cs varies from 600 to 700 A/m.

The petromagnetic section of Mender Mountain (Upper

Cenomanian) can also be divided into 2 complexes. The
lower complex (PC 1) is characterized by relatively high me-
dium size of Jrs (27.5 nT), the upper complex (PC 2) by rela-
tively low size of Jrs (medium—22.1 nT). This section

Fig. 5.

Detailed distribution of carbonate content and petromagnetic parameters in Middle Cenomanian (petromagnetic complex No. 2)

limestone-marl rhythms of Selbuhra section, SW Crimea.

Table l:

Sizes and distribution of studied parameters in the deter-

mined petromagnetic complexes (PC), Middle Cenomanian of Sel-
buhra Mountain.

Jrs

H’cs

Jrs-H’cs

correlation

Variance

of Jrs

7.5–25

high

medium

high

positive

maximal

Extremely
Chaotic
distribution

medium,
oscillation

minimal,
oscillation

Absence
(lower part)
and high
(upper part)

oscillation

oscillated
decrease

minimal

high

negative

increase

minimal

medium

minimal

positive

high

54 GABDULLIN, GUZHIKOV and DUNDIN

(Fig. 6) is characterized by absence of pyrite concretions and
low sizes of dk.

Description of rhytmicity types

In studied area rhythmicity can be divided into 5 types.
First type

. Rhythms of marl-sandy marl; Lower Cenoma-

nian (Mantelliceras mantelli Zone, Inoceramus crippsi Sub-
zone), section 1 (Fig. 2). Boundaries between rhythm ele-
ments are not distinct. There are only small difference
between composition of rhythm elements (Table 2). The
rhythms in the Zong Shan section (Tibet) of the Upper Cen-
omanian (Rotalipora cushmani Zone), could be similar to
this type (Lamocda & Wan 1996).

Second type

. Rhythms of limestone-marl; Upper Ceno-

manian (Rotalipora cushmani, Whiteinella archeocretacea
zones), section 4 (Fig. 6). Boundaries are distinct, bioturbat-
ion is absent, carbonate content decreases towards the top of
the section (to the Cenomanian-Turonian boundary). Thick-
ness of marls strongly varies and increases to the top of the
section (the thickness of the limestone beds is nearly con-
stant). Marls prevail in the section. The composition of
rhythm elements is shown in the Table 3.

Third type

. Rhythms of limestone-marly limestone, marl;

Middle Cenomanian (Rotalipora cushmani Zone), sections 1,
2, 5 (Fig. 3). Boundaries are different: erosive (8); transitional,
diffuse (6); usually distinct. The quantity of pyrite concretions
and carbonate content increase, bioturbation decreases to the
top of the section (Fig. 4). Few rhythms were profoundly
investigated (Fig. 5). Elements of the rhythm show the
difference in content of carbonate, clay, terrigenous minerals,
bioclasts (Tables 4, 5). The planktonic/benthic foraminiferal
ratio is different in the rhythm elements.

Fourth type

. Rhythms of limestone-limestone (due to dif-

ferent carbonate content), Upper Cenomanian (Rotalipora

cushmani Zone), section 3 (Fig. 8). Boundaries are not dis-
tinct, bioturbation is absent or rare, carbonate content increas-
es towards the top of the section (to the Cenomanian-Turonian
boundary). Thickness of rhythm elements strongly varies,
limestones 1 dominate in the section. Their thickness increases
towards the top of the section (the thickness of the limestones
2 is nearly constant). The composition of the rhythm elements
(RE) is shown in the Table 6.

Fifth type.

“Black shale” cyclicity: marly clay-marl

rhythms with 6 horizons of limonite and pyrite concretions.
Upper Cenomanian–Lower Turonian (Rotalipora cushmani,
Whiteinella archeocretacea zones), Aksu-Dere Valley (sec-
tion 6, Fig. 7). Fish remnants in marl couplets were found.
This section was investigated by a group of authors, but the

Fig. 6.

Scheme of correlation, thickness distribution of Upper Cenomanian carbonate rocks (Selbuhra & Mender section) and distribution

of petromagnetic paramaters (Mender section) of Bakhchisarai region, Crimea.

Table 2:

The comparison of rhythm elements (composition, tex-

ture etc.) of the Lower Cenomanian, Selbuhra section.

Marl

Sandy marl

Calcium carbonate, %

65–71

60–70

TOC, %

0.05

Colour

grey

dirty green, grey

Thickness, m

0.4–0.1

0.2–0.4

Bioturbation

medium

Table 3:

The comparison of rhythm elements (composition, tex-

ture etc.) of the Upper Cenomanian, Mender section.

Limestone

Marl

Calcium carbonate, % 81–70

80–65

TOC, %

0.1

0.29

Colour

white

Thickness, m

0.8–1.2

0.3–2.24

Bioturbation

is absent or rare

56 GABDULLIN, GUZHIKOV and DUNDIN

data of analyses, interpretation, field description is some-
times different (Table 7). Shale and adjacent layers can be di-
vided into 6 rhythms (marked by 6 horizons of limonite and
pyrite concretions). Each rhythm consists of a marly clay-
marl alteration. Inside the “shale” two black marls were ob-
served. The presence of 2 black couplets and 6 horizons indi-
cates the maximal anaerobic conditions (no bioturbation).
Other layers were formed in a dysaerobic regime. Data of S,
Mo, Ni, Zn distribution (Gavrilov & Kopaevich 1996) did
not show cyclic fluctuation.

Table 4:

The mineral composition of rhythm elements according to

X-ray diffraction data (PC 1, Middle Cenomanian, Selbuhra
section).

Limestone

Marl

Calcite, %

88.8

69.7

Illite, %

4.5

12.6

Mixedlay, %

0.7

Quartz, %

5.3

8.4

Rutile, %

0.8

Chlorite, %

0.4

Microcline, %

1.4

Montmorillonite, %

7.5

Table 5:

The comparison of rhythm elements (composition, tex-

ture etc.) of the PC 2, Middle Cenomanian, Selbuhra section.

Table 6:

The comparison of rhythm elements (composition, tex-

ture etc.) of the Upper Cenomanian, Selbuhra section.

Limestone

Marly limestone, marl

Carbonate, %

95–70

85–47

TOC, %

0.08

0.44

Colour

white

grey

Thickness, m

0.08–1.3

0.1–0.6

Foraminifera P/B,%

5.5

Carbon isotope 13,%

(Frolov 1996)

20–30

Oxygen isotope 18,%

(Frolov 1996)

–20

–5

Sea water temperature,

degrees centigrade

(Frolov 1996)

23–25

14–15

Ichnofossils & Bioclasts

Limestone 1

Limestone 2

Calcium carbonate, % 95–75

94–70

TOC, %

0.06

0.17

Colour

white

Thickness, m

0.4–4.7

0.1–0.5

Bioturbation

is absent or rare

Table 7:

The comparison of rhythm elements (composition, tex-

ture etc.) of the Upper Cenomanian-Lower Turonian, Aksu-Dere
section.

Marl

Marly clay

TOC, %

(Gavrilov & Kopaevich 1996)

0–9

TOC, % (Naidin & Kyashko 1994) 0–2

0–7

Carbonate, %

(Gavrilov & Kopaevich 1996)

65–70

45–60

Colour

grey, black

pale grey, brown

Texture

Bioturbation

or lamination

L amination

Calcite, % (Alekseev et al. 1997)

60–70

42–60

Quartz, % (Alekseev et al. 1997)

7–25

10–30

Smectite, % (Alekseev et al. 1997) 10–17

10–30

Hydromica, %

(Alekseev et al. 1997)

0–6

3–8

Discussion

There is a group of questions about the origin of rhythmi-

cally bedded Cenomanian carbonate rocks:

(1) Which models explain the origin of the rhythms in pe-

lagic/hemipelagic carbonate rocks?

(2) What mechanisms are responsible for the occurrence of

defined types of rhythmicity?

(3) Which paleogeographical models are suitable for the

studied sections?

(4) Which parts of the basin are represented by studied

sections?

(5) Which agent is responsible for diversity of rhythm

types in the Late Cenomanian?

(6) Why does the succession lose rhythmicity near the

Cenomanian-Turonian boundary?

(7) What is the frequency (time periodicity) of

rhythms?

Different models are suggested to explain the origin of the

rhythmically bedded pelagic/hemipelagic carbonate rocks
(Fig. 9). They can be briefly described.

Dilution cycles. Model 1

(Einsele 1985). Cyclic changes

of moisture, terrestrial input due to climatic variations form
rhythmicity in the carbonate sediments. During a dry climate
mostly limestones are deposited. A wet climate is a time of
marls, with dilution of the constant carbonate sedimentation
by terrigenious material (clay), transported by rivers.

Dilution cycles. Model 2

(Ruffel et al. 1996). This model

is close to the first one. The difference is that in the first case
cyclic climatic changes result in the cyclic changes in vol-
ume of run off, but here climatic fluctuations cause varia-
tions in the nature of weathering and in the composition of
the terrigenious material constantly transported by rivers.
The wet, (or) warm season is a time of marl sedimentation.
Limestones occur during dry, (or) cold conditions.

ORIGIN OF RHYTHMICALLY BEDDED CENOMANIAN CARBONATE ROCKS 57

Dilution cycles. Model 3

(Morozov 1952). Sea level rise

is a time of transgression (ingression) which causes washing
out of accumulated terrestrial material from the shore dis-
tricts into the basin. So, a relatively high terrigenious input
takes place. Sea level fall is a time of regression and relative-
ly low terrestrial input.

Dilution cycles. Model 4

(Gavrilov & Kopaevich 1996).

During sea level fall shore districts become a territory with
swamps and deposition of organic rich sediments. Sea level
rise causes transportation of sediments into the basin (deposi-
tion and partial dissolution, increase of bioproductivity), ap-
pearance of anaerobic conditions and occurrence of “black
shales”.

Solution cycles. Model 5 (

Gabdullin & Baraboshkin

1997). Cyclic repetition of condensation and deposition re-
sult in appearance of rhythmic limestone-carbonate clay
(marl) sections. The limestones always have an erosional
contact with clays (marls). The limestones represent a sedi-
mentation regime, condensation causes the appearance of
carbonate clay, marl (result of limestone dissolution). Ero-
sional surfaces occur due to a non-depositional regime and
include soft- and hard-grounds. Condensation and sedimen-
tation are proposed to be cyclic processes.

Solution cycles. Model 6

(Einsele 1985). SLC causes

variation of the critical carbonate solution depth. Periodical-
ly the solution volume of the constantly deposited carbonate
changes.

Solution cycles. Model 7

(Ricken 1994). Sea level change

causes cyclic depth variation of the basin, which results in pe-
riodic occurrence of stratified waters with anoxic or nearly an-
oxic conditions and solution of the constantly deposited car-
bonates. Sea level up — marl, sea level down — limestone.

Solution cycles. Model 8

(Savdra & Bottjer 1994). Cli-

matic variations result in fluctuations of winds and water
current direction, which cause changes in the content of oxy-
gen dissolved in bottom waters. Because of new current di-
rections and some specific bottom relief forms stagnate,
stratified water masses can occur. Cyclic changes aerobic–
dysaerobic–anaerobic conditions result in periodic solution
of constantly deposited carbonates.

Dilution and solution cycles. Model 9

(Hay 1996). Peri-

odical volcanic input into a basin with mostly carbonate sedi-
mentation causes cyclic appearance of bentonite couplets in-
side chalk or marl layers. Eruption affects in the occurrence
of ash clouds and acid rain. Acid rain enriches the water of
the basin with acids dissolving the carbonate.

Cycles of bioproductivity, dilution, solution. Model 10

(Fischer & Arthur 1977). The whole history of the organic
world can be divided into polytaxon and oligotaxon intervals
(Fischer–Arthur cycles), which occur due to climatic varia-
tions, SLC.

Solution cycles. Model 11

(Einsele 1985). Changes in the

global carbon cycle result in variations of atmospheric chem-
istry and, hence, in climate. The carbon/oxygen relation de-
pends on the volume of vegetation. A higher quantity of
plants causes a lower content of carbon dioxide.

There is no precise data about the frequency and the time

scale of the rhythms presented in the 11 models. It can be
summarized that there are short period rhythms (1, 2) and

long period rhythms (10, 11). The extent of the rhythmic bed-
ding can be local (1–4, 8) and global (9–11).

Five types of rhythms were observed in the investigated

region. We propose models 1–4, 7, 9, 10 to understand the
origin of the studied rhythms.

The first type of rhythm can be interpreted as dilution cy-

cles. Sandy marls rich in benthic fossils (mostly Inoceram-
ides), different quantities of terrigenous material in the
rhythm elements (relatively higher content then in other sec-
tions) show the relatively shallow marine conditions in the
basin of sedimentation which was situated close to the land
(remnants of plants and rare insects). The presence of forests
with insects indicate a warm, wet climate. Diagenetic over-
print is possible. We propose that these rhythms are classi-
fied as dilution cycles (model 1, Fig. 9).

Second type

. Increase of Jrs is thought to be connected

with increase of terrestrial input of magnetite. The type of in-
put (absence of sulphides) is proven by the low sizes of mag-
netic susceptibility and variances of Jrs and H’cs, low size of
Jrs-H’cs correlation. The absence of wide oscillations on the
graph of the Jrs-H’cs correlation was interpreted as nearly
constant dilution (model 2). Increasing thickness of marls
rich in bivalves demonstrates cyclic changes in the carbonate
bioproduction (model 10, Fig. 9). Models 2 and 10 are suit-
able for interpretation of the origin of the second type of
rhythm.

Probably, the same type of rhythm is typical of the Middle

Cenomanian part of the Castagne section, S. Italy (Claps &
Masetti 1994).

Third type

. The estimated sea water temperature (based

on the isotope method) during the time when a limestone bed
accumulated was about 23–25

C. Marls and marly lime-

stones were formed in water with a temperature of 14–15

(Frolov 1996). Marl couplets contain more terrestrial materi-
al, than limestone beds. This can be interpreted as evidence
that a warm, dry climate periodically changed into a cold wet
climate. However, the precise investigation and division of
Middle Cenomanian rocks into 4 petromagnetic complexes
shows a more complicated behaviour of the dilution agent.

Petromagnetic rhythmicity proves dilution cycles in the

Middle Cenomanian section of Selbuhra. Particles of pyrite
and pyrrothine are of allochthonous origin (DTMA data).
This can be supported by low size of Keninberger’s parame-
ter (0.01–0.1). At the same time there is data about authigenic
pyrite in the studied section. High positive Jrs-H’cs correla-
tion can be interpreted as indicating the presence of two Fe-
magnetic minerals. The presence of only one magnetic min-
eral in the rock is defined by diverse (negative) Jrs-H’cs
correlation. A positive correlation with an essential increase
of magnetic susceptibility proves the presence of two miner-
als: pyrite and pyrrhotine. Negative correlation with the ab-
sence of magnetic susceptibility increase indicates the pres-
ence of magnetite. The Middle Cenomanian was a time when
terrestrial input (Table 8) included magnetite (type 1) or
magnetite and sulphides (type 2). The oscillations (positive–
negative) on the graph of Jrs-H’cs correlation (PC 2) are in-
terpreted as dilution cycles. Petromagnetic complexes 1, 3
and 4 are characterized by constant dilution (absence of posi-
tive–negative oscillations).

ORIGIN OF RHYTHMICALLY BEDDED CENOMANIAN CARBONATE ROCKS 59

PC 1

(lower part of the Middle Cenomanian in section 1)

is connected with the second type of constant terrestrial input
of different composition (Table 4). Model number 1 is suit-
able. At the same time solution cycles influenced the sedi-
mentation system. Fluctuations in the oxygen content in the
bottom waters are proven by different concentrations of or-
ganic carbon, pyrite concretions and the volume of bio-
turbated rocks in RE. Ichnofossils Planolites, Zoophycos,
Teichichnus, Chondrites, Thallassinoides, Phycosiphon

ob-

served at the outcrop belong to different bathymetric zones
and cannot coexist. Cyclic distribution of ichnocoenoses
means variations not only in oxygen content but also sea lev-
el change (model 7). Models 1 and 7 are proposed.

PC 2

is characterized by oscillating input of magnetite and

sulphides (dilution cycles). Each subsequent volume of input
was smaller then the previous one. This is proven by constant
decrease of amplitude of oscillation on the graph of Jrs-H’cs
correlation in the direction of the top of the section. We can
conclude that the denudation area was periodically sub-
merged and each successive sea level rise was relatively
higher or the distance to the source constantly increased. It
should be noted that many peaks of dk (up to 5

–6

SI)

indicate the sulphide concentration and are not connected
with the volume of input. Dilution cycles during sea level
change are described by model 3.

PC 3.

Terrestrial constant input of magnetite (type 1). No

sulphides were transported from the land, but increase of dk can
be interpreted as authigenic sulphide deposition caused by
anaerobic–dysaerobic conditions during sea level rise. Erosive
boundaries between rhythm elements were found. Constant di-
lution (model 2) and solution (model 7) took place.

PC 4

is characterized by minimal size of dk and Jrs. It

was a time, when terrestrial input decreased and the basin
was deeper than during the time of accumulation of
sediments of previous petromagnetic complexes. Erosive
boundaries between RE and rare pyrite concretions prove so-
lution cycles. At the top of the PC 4 (top of the Middle Cen-
omanian) a bentonite bed was observed. Sediments of this
petromagnetic complex were accumulated in the conditions
of volcanic dilution and solution. Model 9 is proposed.

Fourth type.

The Upper Cenomanian limestones of Selbu-

hra Mountain (section 3) were divided into two parts. In-
crease of dk without increasing of Jrs can be connected with
authigenic sulphides (pyrite) formed during dysaerobic con-
ditions. Sediments were deposited with terrestrial input of
magnetite (type 1).

PC 1

(lower part of the Upper Cenomanian). Dilution cy-

cles are determined by oscillation of the graph of Jrs-H’cs
correlation and peaks on the graphs of composition of rocks
determined by XRD analysis. Variations in the thickness of
limestone 1 layers (1–4 m) are thought to be caused by bio-
production cycles. The nearly constant P/B relation in RE
(Fig. 8) indicates no change of the sea level. The presence of
one bentonite layer and absence of any solution traces in the
section is unsufficient to support model 9. Proposed models:
1 and 10 (Fig. 9).

PC 2

(upper part of the Upper Cenomanian). This unit is

characterized by the constant dilution (model 2). Variations in
P/B relation (Fig. 8) were interpreted as sea level change. Au-
thigenic sulphides detected by petromagnetic data, rare biotur-
bation and presence of 3 erosional surfaces indicate solution
regime (model 7). Proposed models: 2 and 7 (Fig. 9).

The relatively high medium size of Jrs (18.2 nT) in the Up-

per Cenomanian and relatively low medium size of Jrs (8.1
nT) in the Middle Cenomanian of Selbuhra section (previous
type) can be interpreted as evidence of an increase in the vol-
ume of terrestrial material and a probable increase of tectonic
activity in the basin of sedimentation with gradually increas-
ing depth. In the Early Cenomanian a shallow basin existed
(Fig. 1b) and marl/sandy marl rhythms (type 1) were formed
in it. In the Middle Cenomanian (Fig. 1c) the basin became
deeper and limestone-marl rhythms (type 3) were deposited.
The Late Cenomanian (Fig. 1d) was characterized by the rel-
atively deepest conditions and the appearance of limestone-
limestone rhythms. Tectonic conditions changed at the be-
gining of the second half of the Late Cenomanian from the
relatively stable (lower complex) to relatively unstable (up-
per complex).

Types 3, 4 of the Middle and Upper Cenomanian rhythms

(Crimea) could be similar to the Cenomanian of Scaglia
Bianka Formation: couplets of carbonate rocks with similar
thicknesses of rhythm elements (Schwarzacher 1994).

The fifth type of rhythmicity is thought to be connected

with solution cycles (dysaerobic–anaerobic fluctuations de-
fined by distribution of ichnofossils, rhythms inside “black
shale”, 6 horizons of limonite and pyrite, mineral composi-
tion of rocks, presence or absence of foraminifers) and pos-
sible dilution cycles (cyclic distribution of quartz grains).
Earlier, model 4 (Fig. 9) was proposed for this section
(Gavrilov & Kopaevich 1997). Model 7 (solution cycles) is
also suitable.

Evolution of the Cenomanian sedimentary system

The Lower Cenomanian is presented by shallow deposits

rich in macrofossil invertebrates and marl-sandy marl
succession (Selbuhra section, Figs. 1b, 2). The estimated
depth of the basin containing remnants of plants and insects
was about 40 m. Transgression, started in the Early Cenoma-
nian affected in constant deepening of the basin and Middle
Cenomanian rocks are poor in macrofossils and are presented
by rhythmically bedded limestones and marls (Selbuhra and
Kacha sections, Figs. 1c, 2). Deeper conditions are typical for
Late Cenomanian (Fig. 1d).

Authigenic

minerals

Allochthonous

minerals

Type

of input

Comments

magnetite & sulphides 2

constant input

magnetite & sulphides 2

oscillated input

pyrite

magnetite

constant input

pyrite

Table 8:

Distribution of authigenic and allogenic minerals in the

determined petromagnetic complexes (PC) in Middle Cenomanian
of Selbuhra Mountain.

60 GABDULLIN, GUZHIKOV and DUNDIN

The Upper Cenomanian rocks contained 3 types of rhyth-

micity: marly clay-marl rhythms (Aksu-Dere Valley), lime-
stone-marl rhythms (Mender Mountain), limestone rhythms
(Selbuhra Mountain). The end of the Cenomanian in the stud-
ied region (Aksu-Dere section) is characterized by an oceanic
anoxic event (analogue of “the Bonarelli level”). At the same
time in the Selbuhra section (Upper Cenomanian) the thick-
ness of RE increase up to the first meters (instead of the first
decimeters in the Middle Cenomanian), then rhythmicity dis-
appears or changes into chaotic distribution of layers (2 meters
beyond the Cenomanian-Turonian boundary). Petromagnetic
interpretation of data from the Upper Cenomanian sections
could support the idea of an increase of tectonic activity to-
wards the end of the Cenomanian and a profound change in
the hydrodynamics and depth of the basin. The Selbuhra sec-
tion loses its rhythmicity, the Aksu-Dere section contains
“black shales”, the Selbuhra section (before the arrhythmic in-
terval of the succession) and Mender sections consist of thick
rhythms (1–4.5 m). A.S. Gale found (pers. communication)
syngenetic microfaults in the rocks of the Selbuhra section a
few meters beyond the Cenomanian-Turonian boundary. We
suggest a tectonic factor is responsible for the disappearance
of rhythmicity in the Late Cenomanian. Data derived in this
project is insufficient to make more concrete conclusions
about the nature of the disappearance of rhythmicity at the end
of the Cenomanian.

Bentonitic couplets in the studied region are found in the

Upper Cenomanian and at the boundary between the Middle
and Upper Cenomanian (Selbuhra sections). So, weak
volcanic influence upon sedimentation was possible.

The depth of the basin according to ichnofossils data, fish

data obtained by A.S. Alekseev (Mazarovich & Mileev
1989) varies in the interval of the first hundred meters —
shelf to the shelf edge. It is obvious, that the limestone-marl
rhythms of Mender section rich in macrofossils were
deposited in a shallower part of the basin, than the limestone
rhythms of Selbuhra Mountain. From this point of view the
Upper Cenomanian of the Mender section is close to the
Middle Cenomanian of the Selbuhra section. The relatively
high size of Jrs (up to 40 nT) in the Upper Cenomanian of the
Mender section (Fig. 6) and relatively low size of Jrs (20–30
nT) in the Upper Cenomanian of the Selbuhra section (Fig.
8) can be defined as a relatively high input in the first case
and relatively low — in the second, or the rocks of the
Selbuhra section were deposited in deeper water than the
rocks of the Mender section. The rocks of the Aksu-Dere
section (Fig. 7) presented by “black shale” rich in remnants
of deep marine (sea level rise), were formed in the part of
basin with stratified water masses under an anaerobic
regime. It was a time of deposition of black marls.
Periodically conditions became dysaerobic and marly clays
were deposited.

This described rhythmicity could be the result of the global,

orbital forced cyclic processes — Milankovitch cycles.

The determination of time periodicity of rhythmicity seems

problematic. Firstly, part of the Cenomanian is not exposed
(most of the Lower Cenomanian and base of the Middle Cen-
omanian). Secondly, the Cenomanian of Crimea contains
many erosive boundaries at the base, at the top and inside the

succession. A.S. Gale considers (pers. communication) that
the origin of limestone-marl rhytms of the Middle
Cenomanian (Selbuhra Mountain, type 3) could be connected
with precession cycles.

Conclusions

(1) There are 11 models for the origin of carbonate

rhythms, 7 models are proposed by authors for the studied
sections. Models number 1, 2 and 7 were used three times.

(2, 3) Dilution cycles and possible diagenetic overprint are

responsible for the occurrence of marl-sandy marl rhythms of
the first type (Lower Cenomanian, Mantelliceras mantelli
Zone, Inoceramus crippsi Subzone) in the Selbuhra section
(model 1). Constant dilution, bioproduction cycles caused the
appearance of marl-limestone rhythms of the second type (Up-
per Cenomanian, Rotalipora cushmani Zone) in the Mender
section (models 2, 10). Sections with limestone-marly lime-
stone or marl rhythms of the third type (Middle Cenomanian
Rotalipora cushmani Zone) on the slope of Selbuhra Mountain
were divided into four petromagnetic complexes. PC 1 — cy-
cles of dilution and solution, models 1 and 7. PC 2 — constant
dilution, model 3. PC 3 — constant dilution, solution cycles,
models 2 and 7. PC 4 — volcanic dilution and solution cycles,
model 9. Rhythmic section of the fourth type: limestone-lime-
stone cycles from the Upper Cenomanian (Rotalipora cush-
mani Zone) of Selbuhra Mountain was subdivided into two
petromagnetic complexes. PC 1 — cycles of dilution and bio-
production, models 1 and 10. PC 2 — constant dilution and so-
lution cycles, models 2, 7. “Black shale” cyclicity: marly clay-
marl rhythms with 6 horizons of limonite and pyrite
concretions. Upper Cenomanian-Lower Turonian (Rotalipora
cushmani, Whiteinella archeocretacea zones) marly clay-marl
rhythms from the Aksu-Dere section occurred due to solution
and dilution cycles, models 4 and 7.

(4) The investigated Upper Cenomanian sections repre-

sent different parts of the same basin (distance between out-
crops is about 3–7 km). The shallowest conditions were es-
tablished in the Mender section (limestone-marl rhythms
with complex of benthic fossils). Shallow conditions are
proven for the Selbuhra section (limestone-limestone
rhythms with absence of macrofossils) and depression with
anaerobic–dysaerobic regime or the relatively deeper part of
the basin — Aksu-Dere section (marl-marly clay rhythms
with abundant fish remnants).

(5) The types of rhythms can be classified on the basis of

their lithology and paleogeographical position. Shallow
marine types: 1 — marl-sandy marl (Early Cenomanian);
2 — marl-limestone (Late Cenomanian). Deep marine types:
3 — marl, marly limestone-limestone (Middle Cenomanian);
4 — limestone-limestone (Late Cenomanian); 5 — marl-marly
clay rhythms in black shales (Late Cenomanian). Diversity of
rhythm types in the Late Cenomanian (Mender, Selbuhra,
Aksu-Dere sections) is connected with sedimentation of car-
bonates in different parts of the basin.

(6) A tectonic agent is responsible for the disappearance of

rhythmicity near the Cenomanian-Turonian boundary in the
Selbuhra section. To understand the nature and the driving

ORIGIN OF RHYTHMICALLY BEDDED CENOMANIAN CARBONATE ROCKS 61

force of the tectonic factor more precise investigation must
be done.

(7) The nature of Middle Cenomanian limestone-marl

rhythms could be connected with precession cycles. It
seems problematic to determine the time periodicity of the
investigated rhythms because of the presence of erosive
surfaces (gaps) in the succession, inadequate outcrops and
the unexposed lower part of the Cenomanian section.

Acknowledgments:

We greatly appreciated the assistance of

Alexandr Widrik in taking samples and for help in the lab.
We thank Helmut Weissert for the help and advises. This
work was done with the support of RBS (RFFI, No. 96-05-
65443, 98-05-64196).

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